The present invention relates to cannulas for surgical and medical use. A typical cannula is a fixed diameter tube which a surgeon uses to maintain an instrument passage through tissue to a subcutaneous working location. The surgeon must first make an incision the full depth of the cannula in order to insert the cannula. This traumatic action damages good tissue in order to get to bad tissue. It would be desirable to provide cannulas which do not require a full depth incision, or at least require only a needle-size entrance opening, and which still allow use of a cannula to maintain an instrument passage.
In accordance with one of the features of the invention, cannulas are provided which expand along their entire length. The cannulas are inserted through tissue when in an unexpanded condition and with a small diameter. The cannulas are then expanded radially outwardly to give a full-size instrument passage. Expansion of the cannulas occurs against the viscoelastic resistance of the surrounding tissue. The cannulas may be expanded by inserting members into the cannulas or by fluid pressure.
In accordance with another feature of the invention, a leading end portion of the cannula is constructed to pierce human body tissue. This enables the cannula to form its own opening in body tissue as the cannula is inserted into the tissue.
In accordance with still another feature of the invention, the cannula can be inserted into a blood vessel and expanded. A flow of fluid can be conducted through the cannula into the blood vessel.
The cannula advantageously includes a sheath which encloses a plurality of wires. A member may be inserted into the sheath and moved along the wires to expand the cannula. Each of the wires may advantageously include a core which is at least partially enclosed by a jacket which is integrally formed as one piece with the sheath. If desired, the cannula may be expanded by fluid pressure rather than inserting a member into the cannula.